This is a monstrose (mutant) version of the Blue Myrtle cactus, or Myrtillocactus geometrizans.

It is colloquially known as "The Breast Cactus"... for obvious reasons ;)

Blue Myrtle, or Myrtillocactus geometrizans is native to Central and Northern Mexico. It is a very glaucous (Blue hued) cactus and this one is particularly thick and beautiful. A very popular cactus in landscaping, cultivation, and used as a grafting stock.

This will be a 6-8" cutting, suitable for grafting stock, or a great start for a cultivated plant as well.